The invention concerns a phase-control circuit wherein the output terminal of a phase comparator that compares the actual frequency with the reference frequency is connected to the input terminal of a controlled oscillator.
U. Tietze and Ch. Schenk describe a phase-control circuit, called a phase-locked loop, on pages 683 to 684 of Halbleiter-Schaltungstechnik, 4th ed., Springer, 1978.
Phase-locked loops, often abbreviated PLL, are employed in compact-disk players for example to obtain a synchronizing signal for controlling the speed of a disk drive from the EFM signal, the data signal coded in accordance with an eight-to-fourteen code and optically read from the disk. The speed must be controlled in order to keep the data flowing at a constant bit rate.
One drawback is that the controlled oscillator in the loop must be adjusted to the reference frequency by compensating it manually when the player is manufactured, and manual adjustments are always complicated and expensive.
Another drawback becomes apparent only once the compact-disk player has been operating for a while, when the parameters for the individual components of the loop vary over time as the result of temperature drift or aging of the components and lead to a gradually and initially almost imperceptible deterioration in sound reproduction.